In conventional x-ray systems, a beam of x-rays is directed through an object such as the human body onto a flat x-ray photographic film. The beam of x-rays is selectively absorbed by structures within the object, such as bones within the human body. Since the exposure of the x-ray film varies directly with the transmission of x-rays through the body (and varies inversely with the absorption of x-rays), the image that is produced provides an accurate indication of any structures within the object that absorbed the x-rays. As a result, x-rays have been widely used for non-invasive examination of the interior of objects and have been especially useful in the practice of medicine.
Unfortunately, conventional x-ray systems have their limitations. The image that is formed from the x-ray is basically the shadow of the structures within the object that absorb the x-rays. As a result, the image formed on the x-ray is only two-dimensional, and if multiple x-ray absorbing structures lie in the same shadow, information about some of these structures is likely to be obscured. Moreover, in the case of medical applications, it is often quite difficult to use conventional x-ray systems to examine portions of the body such as the lungs that consist mostly of air when inflated and do not absorb x-rays significantly.
Many of the limitations of conventional x-ray systems are avoided by x-ray computer tomography, which is often referred to as CT. In particular, CT provides three-dimensional views and the imaging of structures and features that are unlikely to be seen very well in a conventional x-ray.
A typical CT apparatus 100 for medical applications is shown in FIG. 1. This apparatus includes a computer 110, a large toroidal structure 120 and a platform 130 that is movable along a longitudinal axis 140 through the center of the toroidal structure. Mounted within the toroidal structure are an x-ray source (not shown) and an array of x-ray detectors (not shown). The x-ray source is aimed substantially at the longitudinal axis and is movable around the interior of the toroidal structure in a plane that is substantially perpendicular to the longitudinal axis. The x-ray detectors are mounted all around the toroidal structure in substantially the same plane as the x-ray source and are aimed at the longitudinal axis. To obtain a CT x-ray image, a patient is placed on the platform and the platform is inserted into the center of the toroidal structure. The x-ray source then rotates around the patient continuously emitting x-rays and the detectors sense the x-ray radiation that passes through the patient. Since the detectors are in the same plane as the x-ray source, the signals they receive relate essentially to a slice through the patient's body where the plane of the x-ray source and detectors intersect the body. The signals from the x-ray detectors are then processed by the computer to generate an image of this slice known in the art as an axial section. Examples of CT axial sections of the thorax are shown in FIGS. 11A-11G.
How this image is generated will be more readily apparent from the simplified explanation of FIG. 2. For purposes of illustration we will consider x-rays emitted from only three points 252, 254, 256 within a toroidal structure 220 in a plane coincident with the plane of the drawing. A platform 240 is movable along an axis 230 perpendicular to the plane of the drawing. Each of points 252, 254, 256 is located on the toroidal structure approximately 120° of arc from the other two points and the beam of x-rays diverges toward the axis 230. An array of x-ray detectors 260 extends around the toroidal structure in the same plane as the x-ray source. If we assume that there is an object on the platform that has an x-ray absorbing feature 270, we can see from FIG. 2 how this feature can be detected and located. The detector array will detect the shadow cast by feature 270 in portions 252A, 254A, and 256A of the x-rays emitted from sources 252, 254 and 256, respectively. However, it will also detect that there was no absorption in regions 252 B&C, 254 B&C and 256 B&C. The failure to detect absorption in regions 252 B&C indicates that the dimension of the feature 270 along the line extending from source 256 to the detectors in the region 256A is simply the projection of region 252A onto that line. Similarly, the dimensions of the feature along the lines from source 252 to region 252A and from source 254 to region 254A can be determined. And from these calculations the shape and location of feature 270 can be determined.
In practice, x-rays are emitted continuously for the full 360° around the patient and numerous features are observed but the overall approach is generally the same.
While the patient remains motionless, the platform is moved along the longitudinal axis through the toroidal structure. In the course of this movement, x-ray exposures are continuously made of the portion of the patient on which CT is to be performed. Since the table is moving during this process, the different x-ray exposures are exposures of different slices of the portion of the patient being examined and the images generated by the computer are a series of axial sections depicting in three dimensions the portion of the patient's body that is being examined. The spacing between adjacent CT sections depends on the minimum size of the features to be detected. For detection at the highest resolution, center-to-center spacing between adjacent sections should be on the order of less than 2 mm.
Because of the superior imaging capabilities of CT, the use of CT in medical imaging has grown rapidly in the last several years due to the emergence of multi-slice CT. However, the cost of conventional CT equipment remains quite high (an average selling price in the United States of $800,000 per unit) and the cost per patient far exceeds the cost of a conventional x-ray.
One application of medical CT is detection and confirmation of cancer. Unfortunately, in all too many cases, this application is merely to confirm the worst. By the time a patient has symptoms enough that warrant the use of CT, the cancer detected by CT has progressed to the point that the patient is almost certain to die of the cancer.
The diagnostically superior information now available in CT axial sections, especially that provided by multidetector CT (multiple slices acquired per single rotation of the gantry) where acquisition speed and volumetric resolution provide exquisite diagnostic value, however, enables the detection of potential cancers at the earliest and most treatable stage. For example, the minimum detectable size of a potentially cancerous nodule in an axial section of the lung is about 2 mm ( 1/10  of inch), a size that is potentially treatable and curable if detected. To intercept a developing cancer in the time between the point at which it first becomes detectable and treatable and the time when it has grown to the point where it is no longer treatable or treatment is pointless; it may become necessary to screen the population at risk on a regular basis. Presently, the standard of care is to find all cancer and potential cancers at their earliest indication. Finding a cost effective way to screen the population for lung cancer remains challenging.
While costs/benefits are such that it is prohibitive to screen the entire population for cancer, there are sub-populations that are at greater risk for cancer than others. One such population is that of present or former smokers. Other such populations are those with occupational exposures to known or suspected carcinogens. For these populations the cost/benefit ratio is such that the use of CT for screening purposes may well be warranted.
Tools that enhance the diagnostic value of the CT scans as well as enable the diagnostic determination by a radiologist in an economically reasonable time are required to assist the physician in the effort to detect cancer at its earliest and most curable stage. These tools are required whether the original examination was performed as a screening or non-screening study.